Effects of land use disturbance on seed banks of riparian forests in southern Manitoba

Riparian forests have been adversely affected by human land use and are threatened across North America. Seed banks play an important role in the maintenance and regeneration of forests, yet effects of land use and fragmentation on forest seed banks remain poorly understood. In 1998 and 1999, we assessed impacts of human disturbance on the diversity and species composition of seed banks in upland portions of riparian forests along an urban-rural gradient in southern Manitoba. Twenty-five forest fragments were categorized according to the following land-use: urban, suburban, high-intensity rural, low-intensity rural, and relatively undisturbed reference classes. Seeds of weedy and exotic species were positively associated with fragmentation, high levels of disturbance, and dry alkaline soils. Seed bank species diversity was lower in urban sites than in rural sites, and the similarity of urban to reference sites was significantly lower than that of rural to reference sites. In contrast, the proportion of exotic to native species richness was highest in seed banks of urban sites. Exotic species Hackelia virginiana and Poa pratense were associated with urban and suburban sites, respectively. Six exotic species were unique to urban sites; these included Hesparis matronalis and Plantago major. In contrast, many of the frequently encountered native species were absent from urban sites; these included Anemone canadensis and Rubus idaeus. These changes in seed bank may affect the ability of riparian forests to recover from adverse impacts associated with urban development and agriculture.